The packaging industry in general finds many applications for multi-layer structures which combine different materials with varying properties designed to provide complex barrier functions.
Metals, particularly aluminum and steel, have been used extensively, typically in the form of cans. As an alternate, aluminum, in the form of foil, has been widely used in other types of packages, especially flexible packaging. Metals protect the contents of a package from external sources of potential contamination such as light, moisture and gases, while simultaneously maintaining a constant atmosphere inside the package by preventing escape of volatile components.
However, there are limitations to the usefulness of metals as barrier materials. One problem is flex cracking, where fractures in the metal defeat barrier properties. Another problem is that one encounters recycling difficulties when a metal foil is disposed within a composite multi-layer structure.
PVDC has been known in the industry as a material by which good barrier properties may be obtained, in lieu of aluminum foil. Aqueous PVDC dispersions or solvent-borne PVDC solutions are easy to apply and form clear, continuous films. However, during the lifetime of a PVDC-containing structure, degradation of the PVDC has been observed. PVDC films discolor upon aging, exposure to light, and exposure to certain agents. When PVDC breaks down, hydrochloric acid is generated, which may then result in acid contamination of the package contents. Further, there has been environmental criticism based upon liberation of hydrochloric acid upon the incineration of multi-layer structures containing PVDC.
EVOH is another polymeric resin that has been shown to provide barrier protection to multi-layer packaging structures. See, for example, Sogi et al U.S. Pat. No. 3,560,325, granted Feb. 2, 1971; and Schroeder U.S. Pat. No. 4,254,169, granted Mar. 3, 1981. EVOH imparts good barrier properties to a structure, and also provides a clear, glossy film. There is no break-down of the polymer to release acid. Nor is there acid contamination of either the package contents or the environment. Films of EVOH are not known to discolor.
EVOH is an attractive alternative to metal foils and PVDC in the packaging industry because it does not exhibit the foregoing disadvantages.
EVOH has typically been applied to composite structures by extrusion coating. However, there are many instances where it is not practical to extrusion coat a substrate with resin. For example, it may be necessary to use one or more adhesive layers to adhere the extruded EVOH to other substrates. This calls for sophisticated, expensive co-extrusion equipment. Further, EVOH should not be processed at temperatures exceeding 500.degree. F. because oxidation and/or degradation of the polymer may take place. Extrusion and co-extrusion expose the resin to elevated temperatures and thus pose a risk of EVOH degradation which results in inferior barrier performance.
EVOH may also be utilized to replace PVDC in multi-layer structures. PVDC is normally applied as a liquid coating. Thus, there is a need in the art for EVOH in liquid form so that it can be applied as a liquid coating.
The prior art reflects various attempts to incorporate EVOH into solutions. Thus, EVOH has been dissolved in mixtures of alcohol and water. However, the resulting solutions are unstable, i.e., the resin precipitates out of solution after a day or so. Another difficulty posed by such solutions is that they do not form very satisfactory films when applied by rod, gravure or similar devices, followed by drying. Instead of obtaining clear and even films, the resulting films exhibit uneven leveling, partial opacity, streaks and voids. Moreover, the solutions must be heated prior to application to a substrate.
The use of a 5-10% solution of an EVOH resin in water, formic acid and isopropanol has also been described in the literature (Modern Plastics, February 1986, page 80) as providing an oxygen barrier by dip coating. It has, however, been found in accordance with the present invention (see Controls E, I and K in the tabulation below) that formic acid- and isopropanol-containing EVOH solutions do not form uniform, high quality EVOH barrier films.
Accordingly, it is among the objects of the present invention to provide an improved EVOH-containing coating solution and a method of coating substrates (e.g., films, pre-formed articles or the like) therewith for forming EVOH barrier coatings for films, rigid containers or other multi-layer structures.